View source for Astronomy:Pan (moon)

{{short description|Moon of Saturn}}
{{for multi|the moon of Jupiter|Carme (moon)|other uses|Pan (disambiguation)}}
{{Infobox planet
| name            = Pan
| image           = Pan color PIA21449.png
| image_scale      =
| caption         = Color photo of Pan by ''Cassini'' in March 2017{{efn|This color photo of Pan was created by combining separate photos taken in infrared, green, and ultraviolet spectral filters of the ''Cassini'' spacecraft's narrow-angle Imaging Science Subsystem (ISS) camera on 7 March 2017. This view of the moon's northern hemisphere shows its [[Biology:Walnut|walnut]]-like appearance, with a highly inclined equatorial ridge almost eclipsing the moon's southern hemisphere from view.}}
| discovered      = July 16, 1990
| pronounced      = {{IPAc-en|ˈ|p|æ|n}}
| named_after     = Πάν ''Pān<!--the 'a' is long in Greek, as reflected in the Latin, even if not written in Greek-->''
| mpc_name        = Saturn XVIII
| alt_names       = S/1981 S 13
| adjectives      = Pandean {{IPAc-en|p|æ|n|ˈ|d|iː|ə|n}}<ref>{{OED|Pandean}}</ref>
| discoverer      = [[Biography:Mark R. Showalter|M. R. Showalter]]
| orbit_ref       = <ref name="Jacobson2008">{{cite journal | author= Jacobson, R. A. | display-authors=0| title= Revised orbits of Saturn's small inner satellites| journal=Astronomical Journal| date= 2008| volume= 135 | issue= 1| pages= 261–263| bibcode=2008AJ....135..261J | doi= 10.1088/0004-6256/135/1/261| citeseerx= 10.1.1.653.3917| s2cid= 122998668}}</ref>
| semimajor       = {{val|133584.0|0.1|u=km}}
| eccentricity    = {{val|0.0000144|0.0000054}}
| period          = {{val|0.575050718|u=days}} ({{val|13.801217|u=hours}})
| inclination     = {{val|0.0001|0.0004|u=°}}
| satellite_of    = [[Astronomy:Saturn|Saturn]]
| group           = [[Astronomy:Shepherd moon|Shepherd moon]] of the Encke Gap
| dimensions      = {{val|34.6|x|28.2|x|21.0}}&nbsp;km<br/>(±&nbsp;{{val|0.4|x|0.4|x|1.0}}&nbsp;km)<ref name="Thomas2020"/>{{rp|page=2}}
| mean_diameter   = {{val|27.4|0.6|u=km}}<ref name="Thomas2020"/>{{rp|page=2}}
| volume          = {{val|10748|u=km3}}<ref name="Thomas2020"/>{{rp|page=8}}
| mass            = {{val|4.30|0.22|e=15|u=kg}}<ref name="Thomas2020"/>{{rp|page=3}}
| density         = {{val|0.400|0.031|u=g/cm3}}<ref name="Thomas2020"/>{{rp|page=3}}
| surface_grav    = {{val|0.0111|-|0.0169|u=m/s2}}<ref name="Thomas2020"/>{{rp|page=3}}
| escape_velocity = {{V2|4.30e-6|17.3|3}}&nbsp;km/s at longest axis<br/>to {{V2|4.30e-6|10.5|3}}&nbsp;km/s at poles<!-- V2 template uses mass in terms of e+21 kg, radius in terms of km, and number of decimal places -->
| rotation        = synchronous
| axial_tilt      = zero
| albedo          = 0.5
| single_temperature = ≈&thinsp;78 K
}}

'''Pan''' is the innermost named [[Astronomy:Natural satellite|moon]] of [[Astronomy:Saturn|Saturn]].<ref name="Moons of Saturn">{{cite web |url=https://solarsystem.nasa.gov/planets/saturn/moons |archive-url=https://web.archive.org/web/20150907020115/http://solarsystem.nasa.gov/planets/saturn/moons |url-status=dead |archive-date=7 September 2015 |title=Saturn - Moons |work=NASA |access-date=5 October 2017}}</ref> It is a small, [[Biology:Walnut|walnut]]-shaped moon approximately 35 [[Kilometre|kilometre]]s across and 23&nbsp;km wide that orbits within the Encke Gap in Saturn's A Ring. Pan is a [[Astronomy:Shepherd moon|ring shepherd]] and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a ravioli.<ref name="Ravioli">{{cite web |url=https://www.cnet.com/news/nasa-saturn-pan-cassini-ravioli-moon/ |title=Saturn's Tiny Moon Pan Looks Like a Ravioli |work=CNET |access-date=8 January 2020}}</ref>

It was discovered by [[Biography:Mark R. Showalter|Mark R. Showalter]] in 1990 from analysis of old ''Voyager 2'' probe photos and received the provisional designation '''{{nowrap|S/1981 S 13}}''' because the discovery images dated back to 1981.<ref name="iauc5052">[http://www.cbat.eps.harvard.edu/iauc/05000/05052.html IAUC 5052: ''Saturn''] July 16, 1990 (discovery)</ref>

==Prediction and discovery==
The existence of a moon in the Encke Gap was first predicted by Jeffrey N. Cuzzi and Jeffrey D. Scargle in 1985, based on wavy edges of the gap which indicated a gravitational disturbance.<ref>Cuzzi, J. N.; and Scargle, J. D.; [http://adsabs.harvard.edu//full/seri/ApJ../0292//0000276.000.html ''Wavy Edges Suggest Moonlet in Encke's Gap''], Astrophysical Journal, Vol. 292 (May 1, 1985), pp. 276–290</ref> In 1986 Showalter ''et al.'' inferred its orbit and mass by modeling its gravitational wake. They arrived at a very precise prediction of 133,603 ± 10&nbsp;km for the semi-major axis and a mass of 5&ndash;10{{e|&minus;12}} Saturn masses, and inferred that there was only a single moon within the Encke gap.<ref name="Showalter1986">{{cite journal | author= Showalter, M. R. | display-authors=0| title= Satellite "wakes" and the orbit of the Encke Gap moonlet| journal= Icarus| date= 1986| volume= 66 | issue= 2| pages= 297–323| bibcode= 1986Icar...66..297S | doi = 10.1016/0019-1035(86)90160-0| url= https://zenodo.org/record/1253894| doi-access= free}}</ref> The actual semi-major axis differs by 19&nbsp;km and the actual mass is 8.6{{e|&minus;12}} of Saturn's.

The moon was later found within 1° of the predicted position. The search was undertaken by considering all ''[[Astronomy:Voyager 2|Voyager 2]]'' images and using a computer calculation to predict whether the moon would be visible under sufficiently favorable conditions in each one. Every qualifying Voyager 2 image with resolution better than ~50&nbsp;km/pixel shows Pan clearly. In all, it appears in eleven ''Voyager 2'' images.<ref name="Showalter1990">{{cite journal | author=Showalter, M. R.| title= Visual Detection of 1981 S 13, the Encke Gap Moonlet| journal=  Bulletin of the American Astronomical Society| date= 1990| volume= 22| pages= 1031| url= http://adsabs.harvard.edu//full/seri/BAAS./0022//0001040.000.html}}</ref><ref name="Showalter1991">{{cite journal | author= Showalter, M. R.| title= Visual detection of 1981 S 13, Saturn's eighteenth satellite, and its role in the Encke gap| journal= Nature| date= 1991| volume= 351 | issue= 6329| pages= 709–713| doi=  10.1038/351709a0|bibcode = 1991Natur.351..709S | s2cid= 4317496}}</ref>

==Name==
The moon was named on 16 September 1991,<ref>[http://www.cbat.eps.harvard.edu/iauc/05300/05347.html IAUC 5347: ''Satellites of Saturn and Neptune''] 1991 September 16 (naming the moon)</ref> after the mythological Pan, who was (among other things) the god of shepherds.  This is a reference to Pan's role as a [[Astronomy:Shepherd moon|shepherd moon]].  It is also designated '''{{nowrap|Saturn XVIII}}'''.<ref name="Gazetteer">{{cite web|title = Planet and Satellite Names and Discoverers|work = Gazetteer of Planetary Nomenclature|publisher = USGS Astrogeology|date = July 21, 2006|url = http://planetarynames.wr.usgs.gov/append7.html|access-date = 2006-08-07 }}</ref>

==Orbit==
The eccentricity of Pan's orbit causes its distance from Saturn to vary by ~4&nbsp;km.  Its inclination, which would cause it to move up and down, is not distinguishable from zero with present data.  The Encke Gap, within which Pan orbits, is about 325&nbsp;km wide.

==Geography==
[[File:Pan (moon) edge-on.jpg|thumb|right|Pan, photographed by ''[[Astronomy:Cassini–Huygens|Cassini]]'' on March 7, 2017. The thin equatorial ridge is clearly visible.]]
''Cassini'' scientists have described Pan as "[[Biology:Walnut|walnut]]-shaped"<ref>[http://photojournal.jpl.nasa.gov/catalog/PIA08320 "PIA08320: Cruising with Pan"], ''Planetary Photojournal''.</ref> owing to the equatorial ridge, similar to that on [[Astronomy:Atlas (moon)|Atlas]], that is visible in images. The ridge is due to ring material that Pan has swept up from the Encke gap. It has been referred to by journalists as a space empanada, a form of stuffed bread or pastry, as well as a ravioli.<ref>{{cite news|last1=Chang|first1=Kenneth|title=Pan, Moon of Saturn, Looks Like a Cosmic Ravioli (or Maybe a Walnut)|url=https://www.nytimes.com/2017/03/10/science/nasa-cassini-saturn-moon-pan.html?_r=0|access-date=11 March 2017|work=The New York Times|date=10 March 2017}}</ref><ref name="Space empanada">{{Cite web |url=http://www.sciencemag.org/news/2017/03/space-empanada-frozen-ridge-around-saturn-s-moon-pan-collected-planet-s-rings |title= Stunning close-up of Saturn's moon, Pan, reveals a space empanada |last=Perkins |first=Sid |website=Science |date=2017-03-09}}</ref> A new study suggests that the bizarre shape of Pan could also be due to collisions between tiny [[Astronomy:Moonlet|moonlets]], thus causing them to merge and form Pan (known as the pyramidal regime formation scenario).<ref>{{Cite web|last=|first=|date=22 May 2018|title=Here's Why Saturn's Inner Moons Are Shaped Like Ravioli and Potatoes|url=https://www.space.com/amp/40653-saturns-weird-moon-shapes-explained.html|access-date=8 February 2021|website=Space.com}}</ref>

==Pandean ringlet==
The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in [[Astronomy:Horseshoe orbit|horseshoe orbit]]s.<ref>{{cite journal|last1=Hedman|first1=M.M.|last2=Burns|first2=J.A.|last3=Hamilton|first3=D.P.|last4=Showalter|first4=M.R.|title=Of horseshoes and heliotropes: Dynamics of dust in the Encke Gap|journal=Icarus|date=2013|volume=223|issue=1|pages=252–276|doi=10.1016/j.icarus.2012.11.036|arxiv=1211.4762|bibcode=2013Icar..223..252H|s2cid=974916}}</ref> A second ringlet is periodically disrupted by Pan, similarly to how the F Ring is disturbed by [[Astronomy:Prometheus (moon)|Prometheus]].<ref>{{cite journal|last1=Porco|first1=C.C.|last2=Baker|first2=E.|last3=Barbara|first3=John|last4=Beurle|first4=K.|last5=Brahic|first5=A.|last6=Burns|first6=J.A.|last7=Charnoz|first7=S.|last8=Cooper|first8=N.|last9=Dawson|first9=Douglas|last10=Delgenio|first10=Anthony|last11=Denk|first11=T.|last12=Dones|first12=Luke|last13=Dyudina|first13=Ulyana|last14=Evans|first14=M.W.|last15=Giese|first15=B.|last16=Grazier|first16=Kim|last17=Helfenstein|first17=Paul|last18=Ingersoll|first18=A.P.|last19=Jacobson|first19=R.A.|last20=West|first20=Robert|title=Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites|journal=Science|date=2005|volume=307|issue=5713|pages=1226–1236|doi=10.1126/science.1108056|pmid=15731439|bibcode=2005Sci...307.1226P|s2cid=1058405}}</ref>

==Gallery==
<gallery>
File:PIA06534 Encke Division.jpg|Closeup of the Encke Gap, showing the central ringlet that is coincident with Pan's orbit.
File:Pans highway 3-05-07.jpg|''Cassini'' image showing Pan orbiting in the Encke Gap.
File:Pan side view.jpg|Equatorial view of Pan from ''Cassini'', with the rings of Saturn surrounding the moon.
File:Pan Rev09.2x.jpg|Pan in the center of the image, occupying the Encke Gap in Saturn's rings. Its walnut-like shape is clearly visible.
File:PIA21436 - Pan, 3 versions.jpg|Stretched (4x) images, processed in various ways.
</gallery>

==See also==
*[[Astronomy:List of natural satellites|List of natural satellites]]

=== Notes ===
{{notelist}}

== References ==
{{reflist|30em|refs=

<ref name="Thomas2020">{{cite journal
  |first1 = P. C. |last1 = Thomas
  |first2 = P. |last2 = Helfenstein
  |title      = The small inner satellites of Saturn: Shapes, structures and some implications
  |journal    = Icarus
  |date       = July 2020
  |volume     = 344
  |id         = 113355
  |pages      = 20
  |doi        = 10.1016/j.icarus.2019.06.016
  |bibcode    = 2020Icar..34413355T|s2cid = 197474587
 }}</ref>

}}

== External links ==
* [https://web.archive.org/web/20070609080459/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Pan Pan Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [https://web.archive.org/web/20060927042138/http://www.planetary.org/explore/topics/our_solar_system/saturn/pan.html The Planetary Society: Pan]

{{Moons of Saturn|state=uncollapsed}}
{{Saturn}}
{{DEFAULTSORT:Pan (Moon)}}
[[Category:Moons of Saturn]]

{{Sourceattribution|Pan (moon)}}